With the constantly improving quality of color photocopies and printings and in an attempt to protect security documents such as banknotes, value documents or cards, transportation tickets or cards, tax banderols, and product labels against counterfeiting, falsifying or illegal reproduction, it has been the conventional practice to incorporate various security means in these documents. Typical examples of security means include security threads or stripes, windows, fibers, planchettes, foils, decals, holograms, watermarks, security inks comprising optically variable pigments, magnetic or magnetizable thin film interference pigments, interference-coated particles, thermochromic pigments, photochromic pigments, luminescent, infrared-absorbing, ultraviolet-absorbing or magnetic compounds.
Security threads embedded in the substrate are known to those skilled in the art as an efficient means for the protection of security documents and banknotes against imitation. Reference is made to U.S. Pat. No. 0,964,014; U.S. Pat. No. 4,652,015; U.S. Pat. No. 5,068,008; U.S. Pat. No. 5,324,079; WO 90/08367 A1; WO 92/11142 A1; WO 96/04143 A1; WO 96/39685 A1; WO 98/19866 A1; EP 0 021 350 A1; EP 0 185 396; EP 0 303 725; EP 0 319 157 A1; EP 0 518 740 A1; EP 0 608 078 A1; EP 0 635 431 A1; and EP 1 498 545 A1 as well as the references cited therein.
A security thread is a metal or plastic filament, which is incorporated during the manufacturing process into the substrate serving for printing security documents or banknotes. Security threads or stripes carry particular security elements, serving for the public- and/or machine-authentication of the security document, in particular for banknotes. Common types of security thread include metal-formed characters or indicia disposed on a plastic substrate. Such threads, which are coated with a very thin layer of metal, such as aluminum, and then demetalized, either display discrete metal characters or negative or reverse-image characters. With the aim of further increasing the resistance against counterfeit or illegal reproduction of security threads, it has been a practice to incorporate additional security features within the structure of said threads. Security features, e.g. for security documents, can generally be classified into “covert” security features on the one hand, and “overt” security features on the other hand. The protection provided by covert security features relies on the concept that such features are difficult to detect, typically requiring specialized equipment and knowledge for detection, whereas “overt” security features rely on the concept of being easily detectable with the unaided human senses, e.g. such features may be visible and/or detectable via the tactile senses while still being difficult to produce and/or to copy. Typical examples of additional security features for security threads include optically variable materials, luminescent materials, IR absorbing materials and magnetic materials.
WO 2004/048120 discloses security elements comprising at least two adjacent regions, wherein one of the regions is an optically variable layer and the other region has a layer of material with constant reflection. The disclosed security element comprises regions forming areas without material in order to form graphic makings, characters and the like that can be detected visually.
U.S. 2007/0241553 discloses security elements for securing valuable articles having an optically variable layer that imparts different color impressions at different viewing angles and, in a covering area, a semi-transparent ink layer disposed on top of the optically variable layer, the color impression of the optically variable layer being coordinated with the color impression of the semi-transparent ink layer in the covering area when viewed under predefined viewing conditions.
U.S. 2011/0095518 discloses security elements for securing valuable articles comprising a stacked layer made of an optically variable layer that conveys different color impressions at different viewing angles, and a color-constant layer comprising an ink layer and a metal layer. The optically variable layer and the color-constant layer are stacked in a covering region, while at most one of the optically variable layer and the color-constant layer is present outside the covering region. The color impression of the stacked layers in the covering region and the color impression of the one layer outside the covering region are matched with each other when viewed at a predetermined viewing angle.
EP 2 465 701 A2 discloses security elements for securing valuable articles comprising a stacked layer made of an optically variable layer that conveys different color impressions at different viewing angles, a first portion with a first color-constant impression and a second portion with a color-constant impression and an individualizing marking. The optically variable layer and the two portions exhibiting two color-constant impressions are stacked in a covering region. The disclosed different layers are coordinated so that the color impression of the optically variable layer matches at a predetermined first viewing angle the color impression of the first portion and that the color impression of the optically variable layer matches at a predetermined second viewing angle being different from the first viewing angle the color impression of the second portion.
Magnetic materials have been used as machine readable security features in security threads. Unfortunately, these materials have a degree of inherent color, which renders them visually detectable in reflected or transmission light through a surface of a security paper. Attempts have therefore been made to hide or conceal these materials. While magnetic regions are not visually discernable, the counterfeiter will not be able to reproduce them and therefore the counterfeiting will fail and/or will be easily detected.
CA 2,076,532 C discloses security threads comprising a metallic layer with recesses in the form of characters or patterns and magnetic areas in regions which do not comprise the metallic layer. The magnetic areas of the security threads described in CA 2,076,532 C are not visible by being hidden by the metallic layer.
EP 0 310 707 A2 discloses security threads or stripes comprising magnetically detectable and readable anti-forgery and/or anti-fraud means. The disclosed security threads or stripes comprise mutually spaced magnetic regions obtained with a deposition of magnetic material such as for example magnetic iron oxide. EP 0 310 707 A2 further discloses that a masking layer may be further added so as to hide the magnetic regions from view and thus prevent the fraudulent tampering or reproduction of said regions.
U.S. Pat. No. 6,549,131 discloses methods for camouflaging or burying magnetic machine readable information by using one or more metalized foil layers.
However, the incorporation of a metalized layer to hide the magnetic areas may result in the deterioration of the security thread or stripe upon use and time due to the potential corrosion of the metalized layer. To overcome such a deterioration, additional layers acting as corrosion resistant layers may generally be used.
EP 1 497 141 B1 discloses security substrates comprising a transparent polymer carrier layer bearing indicia formed from a plurality of metalized and demetalized and a clear and transparent magnetic layer, wherein said magnetic layer contains particles of a soft magnetic material of a size in a concentration and size distribution at which the magnetic layer remains clear and transparent.
However, the combination of magnetic layers with metalized layers as well as hiding layers and corrosion resistant layers lead to highly thick security threads which may cause difficulties during the integration of said threads in paper.
There remains a need for sophisticated machine readable security threads or stripes combining a high resistance against counterfeiting or illegal reproduction of security documents comprising said security threads or stripes with a machine readable magnetic code which is not visually detectable in the absence of an additional hiding layer. Said security threads or stripe could thus make impossible the reproduction of said security threads or stripes without knowing in advance said magnetic code while said security threads or stripes have a thickness allowing their incorporation in/on a security document such as banknote.